1. Field of the Invention
The present invention relates broadly to apparatus and methods for investigating subsurface earth formations. More particularly, this invention relates to borehole tools and methods for making hydraulic property measurements of an earth formation surrounding a borehole, and in particular apparatus and methods for generating appropriate pressure-pressure response functions in homogeneous and heterogeneous formations.
2. State of the Art
The determination of permeability (fluid mobility) and other hydraulic properties of formations surrounding boreholes is very useful in gauging the producibility of formations, and in obtaining an overall understanding of the structure of the formations. For the reservoir engineer, permeability is generally considered a fundamental reservoir property, the determination of which is at least equal in importance with the determination of porosity, fluid saturations, and formation pressure. When obtainable, cores of the formation provide important data concerning permeability. However, cores are difficult and expensive to obtain, and core analysis is time consuming and provides information about very small sample volumes. In addition, cores, when brought to the surface, may not adequately represent downhole conditions. Thus, in situ determinations of permeability which can quickly provide horizontal and vertical determinations of permeabilities over larger portions of the formation are highly desirable.
Existing techniques for making permeability determinations can be classified into indirect and direct methods. In indirect methods, permeability is determined from empirical correlations which attempt to express permeability in terms of other measured formation parameters, such as porosity, saturation, or mineralogy. A direct measurement technique involves actual measurement of fluid flow, pressure, etc. and determination of permeability from these measurements.
Suggestions regarding a direct in situ determination of permeability via the injection or withdrawal of fluid into or from the formation and the measurement of pressures resulting therefrom date back at least to U.S. Pat. No. 2,747,401 to Doll (1956). In the Doll patent there is disclosed a method and apparatus for determining hydraulic characteristics, including permeability, fluid pressure, and hydraulic anisotropy, of formations surrounding a borehole. A pressure gradient is obtained in the formations by pressing or pushing a probe against the borehole wall. Pressure differences between different points are then used to obtain indications of hydraulic characteristics of the formations. In an embodiment disclosed in the patent, a pair of spaced probes are pressed against the formation, and a pressure gradient is generated by injecting a fluid into the formation at one of the probes (a source probe) at a constant flow rate. The other probe (a measurement probe) is coupled to a pressure responsive device. Pressure is measured at the measurement probe before and after injection of the fluid at the source probe. The permeability of the formation is then obtained using a formula in which permeability is proportional to viscosity times flow rate divided by the change in pressure. The patent points out that the pressure gradient can also be obtained by extracting fluid from the formation and that measurements can be made in more than one direction; e.g., vertical and horizontal, to obtain indications of both vertical and horizontal hydraulic characteristics.
Different devices have been used for making direct measurements of permeability. For example, devices whose primary use has been for sampling formation fluids, have also been used with some success in estimating formation permeability. Formation testing devices which can take repeated samples are disclosed, for example, in U.S. Pat. No. 3,780,575 to Urbanosky and U.S. Pat. No. 3,952,588 to Whitten, both of which are hereby incorporated by reference herein in their entireties. In these devices, a hydraulic pump provides pressure for the operation of various hydraulic systems in the device. Sample chambers are provided in the tool to take samples of formation fluid by withdrawing hydraulically operated pistons. Pressure transducers are provided to monitor pressure as the fluid is withdrawn, and pressure can be continuously recorded. So-called pre-test chambers are also typically provided and are operated to permit more reliable flow during the subsequent fluid withdrawal. Filters can also be provided to filter sand and other particulate matter, and pistons can be provided to clean the filters, such as when the tool is retracted.
One type of formation testing device includes an elongated body and a setting arm activated by setting pistons which are used to controllably urge the body of the device against a side of the borehole wall at a selected depth. The side of the device that is urged against the borehole wall includes a packer which surrounds a probe. As the setting arm extends, the probe is inserted against the formation, and the packer then sets the probe in position and forms a seal around the probe, whereupon the fluids can be withdrawn from the formation during pre-test and the actual test.
The primary technique presently used for in situ determination of permeability is the “drawdown” method where a probe of a formation testing tool is placed against the borehole wall, and the pressure inside the tool (e.g., at a chamber) is brought below the pressure of the formation, thereby inducing fluids to flow into the formation testing tool. By measuring pressures and/or fluid flow rates at and/or away from the probe, and processing those measurements, determinations regarding permeability are obtained. These determinations, however, have typically been subject to large errors. Among the reasons for error include the fact that if fluid is extracted at a fixed flow rate which is independent of permeability, as is typically done, in low permeability formations the pressure drop tends to be too large, and solution gas and/or water vapor forms and can make the results uninterpretable. Indeed, liberation of gas during drawdown provides anomalous pressure and fluid flow rate readings. Another source of error is the damage to the formation (i.e., pores can be clogged by migrating fines) which occurs when the fluid flow rate towards the probe is caused to be too large. See, e.g., T. S. Ramakrishnan et al., “A Laboratory Investigation of Permeability in Hemispherical Flow with Application to Formation Testers”, SPE Form. Eval. 10, pp. 99–108 (1995).
More recent patent disclosures of permeability testing tools include U.S. Pat. No. 4,742,459 to Lasseter, and U.S. Pat. No. 4,860,581 to Zimmerman et al. (both of which are hereby incorporated by reference herein in their entireties) which further develop the draw-down techniques. In the Lasseter patent, a logging device is provided having a source probe, a horizontal observation probe which is azimuthally displaced on the borehole wall with respect to the source probe position, and a vertical observation probe which is vertically displaced on the borehole wall with respect to the source probe position. The source probe is provided with means for withdrawing fluid at a substantially constant rate or pressure, while the vertical and horizontal probes, as well as the source probe, are provided with means for measuring formation pressure response as a function of time. According to the method for determining permeability, a transient pressure change is established in the formation by withdrawing fluid from the formation at the source probe location. The formation pressure response is then measured at the vertical and horizontal probes. By selecting a trial permeability value, theoretical formation pressure responses can be derived as a function of time at the probe locations. The theoretical formation pressure responses are then compared with the actually measured pressure responses in an iterative manner, with the difference being used as feedback to modify the trial value, until the difference is negligible.
The Zimmerman et al. patent mentions that in the drawdown method, it is essential to limit the pressure reduction so as to prevent gas liberation. In order to prevent gas liberation, Zimmerman et al. propose a flow controller which regulates the rate of fluid flow into the tool.
Additional progress in in situ permeability measurement is represented by U.S. Pat. No. 5,269,180 to Dave et al., U.S. Pat. No. 5,335,542 to Ramakrishnan et al., and U.S. Pat. No. 5,247,830 to Goode, all of which are hereby incorporated by reference herein in their entireties. In the Dave et al. patent, borehole tools, procedures, and interpretation methods are disclosed which rely on the injection of both water and oil into the formation whereby endpoint effective permeability determinations can be made. In the Ramakrishnan et al. patent, a tool which integrates hydraulic and electromagnetic measurements (images) is disclosed. In the Goode patent, methods are disclosed for making horizontal and vertical permeability measurements without the necessity for measuring flow rate into or out of the borehole tool. In particular, an interpretation scheme is presented in which the change in pressure at the vertical observation probe is related to the change in pressure at the horizontal probe through a convolutional integral. The kernel function G in this integral is independent of the flowrate at the sink probe. This scheme is called pressure-pressure deconvolution, and it eliminates the need for knowing the tool storage volume (i.e., the volume of fluid in the tool connected to the formation) and the formation damage at the sink probe. However, the problem of storage at the observation probes remains and can be a source of error in the interpretation because the local flow at each observation probe causes a pressure change that cannot be neglected. Thus, even with these inventions which have advanced the art significantly, the accuracy and scope of the information obtained is not to the level desired.